The investigators propose a randomized non-inferiority trial that compares preoperative Fluoro Uracil (FU)-based chemoradiotherapy to radiotherapy with a simultaneous integrated boost. In patients with T3-4 rectal cancer, the latter approach is considered preferential with regard to toxicity and cost. The metabolic response of the tumor, as assessed by 18F-2-Fluoro-2-Deoxyglucose-Positron Emission tomography (18F-FDG PET) or PET-CT, will be used as a surrogate marker of cause specific outcome